This invention is in the field of wireless computer networks, and is more specifically directed to the optimization of data transmissions in wireless networks.
In recent years, the networking of multiple computers has become widespread, not only in business environments but also in the home. While many of these Local Area Networks (LANs) are “wired” networks, such as Ethernet networks, wireless networks have recently become quite popular, in large part because of recent reductions in the cost of wireless network equipment. An attractive benefit of wireless networks is the ease of installation, because cabling need not be run in order to set up a network. In addition, portable computing devices, such as laptop personal computers, become truly portable when communicating over wireless networks. Indeed, many businesses such as coffee shops, airports, convention centers, and hotels, are now deploying wireless networks at their locations, to provide Internet and email service to their visiting customers. Many schools are now also deploying wireless LANs in the classroom, permitting students with laptop computers and wireless network adapters to download and upload assignments, class notes, and the like, and to communicate with the instructor and with other students.
Communications standards have been an important factor in the widespread deployment of wireless networks. Current wireless networking standards include the 802.11b and 802.11a standards, both promulgated under the auspices of the Institute of Electrical and Electronics Engineers, Inc. (IEEE). Many computing and networking devices now operate according to these standards, permitting widespread use of this technology. By way of further background, the wireless networking industry is now working toward a new, higher data rate, wireless LAN standard, commonly referred to as 802.11 g. Under this standard, it is expected that high data rates, up to and perhaps exceeding the 802.11a data rate of 54 Mbps, will be implemented on the 2.4 GHz frequency band. The 802.11a, 802.11b, and 802.11 g standards are all physical layer standards, each of which can operate in connection with the current 802.11 media access control (MAC) standard, or with the enhanced standard referred to as 802.11e, which implements Quality of Service (QoS) capability.
As is well known in the wireless LAN art, one complication in the operation of a wireless network is the management of reliable communications among the various elements of the network within a finite bandwidth. Typically, multiple wireless network elements (e.g., laptop computers) are in communication with one another, with at least one of the network elements being a wireless access point that in turn connects to a network backbone; alternatively, especially in the home or small office environment, the wireless access point connects to a wide area network (WAN), such as the Internet, through a modem. In each case, communications may be carried out among the multiple network elements themselves, and also upstream from the access point to and from locations on a larger network or on the Internet. For reliable communications, of course, communications to and from each individual network element must not collide or interfere with communications to and from the other network elements.
In conventional 802.11 wireless communications, the various network elements all share the same frequencies, operating either according to a frequency-hopping scheme or Direct-Sequence Spread-Spectrum (DSSS) technology. In frequency-hopping, transmissions change frequency within a finite bandwidth over time, following a pseudo-random pattern. Wireless networks in the United States that follow the 802.11 standard utilize DSSS, in which each data bit is spread by a pseudo-random code. In effect, DSSS code spreading spreads the transmission over a wider band of frequencies, and provides noise immunity by incorporating inherent redundancy in the transmitted signal. In either case, as mentioned above, the wireless network elements within a service area share the same frequencies, both for transmission and receipt. In the event that transmissions within the frequency band collide, the message “packets” or “frames” that are corrupted by the collision are retransmitted. Collisions therefore reduce the effective data rate of the network. Conversely, idle time spent by the network elements in order to avoid collisions also reduce the effective data rate of the network.
According to conventional approaches, collision avoidance is carried out among the various network elements themselves, in combination with the wireless access point. According to a conventional approach, of the type referred to as non-persistent Carrier Sense Multiple Access (CSMA), each station within a service set (i.e., area served by one or more wireless access points) of the network listens for traffic from all other users in that service set, and waits until the network channel is idle before transmitting. While the channel is busy, each station that is ready to transmit a frame waits until the channel is clear before attempting its own transmission.
Fairness in assigning the next available transmission time is determined, under the 802.11 standard, by each station waiting for a pseudo-randomly selected period after the completion of a prior frame, before beginning its own frame transmission. In this conventional approach, each network element loads a counter with a pseudo-randomly selected value within a determined interval (0, CW], and begins counting down from this selected value after the prior frame is transmitted. Typically, the counting down begins after the acknowledgment by the destination network element of the receipt of a valid frame, followed by an interframe space referred to as the Distributed Interframe Space (DIFS). When the DIFS is over, each station waiting to transmit begins counting down from its random value. Upon a station's counter reaching zero, that station then begins transmission of its frame over the channel. The randomness in the setting of the counter ensures fairness among the various network elements in the service set. Of course, collisions may still occur, for example by a second station beginning transmission prior to its detecting that another station has already begun transmission. According to this conventional approach, if a collision occurs, the upper limit CW of the pseudo-random range is increased (typically doubled) at each station, improving the chances for a successful transmission, but at a cost of additional latency on the channel.
The upper limit CW of the pseudo-random value range varies from a minimum value CWmin to a maximum value CWmax. Typically, the upper limit CWmax is the capacity of the counters in the stations. According to conventional approaches, such as under the 802.11 standard, the minimum value CWmin is fixed. For example, minimum value CWmin has the value 31 for 802.14b networks, and the value 15 for 802.11a networks. By way of further background, it is believed that, under the draft 802.11e standard, minimum value CWmin will be a programmable value that depends upon the particular traffic class, and that will be set by a broadcast information element.
By way of still further background, it is known in the art that the data throughput of a wireless network over a measurement period is maximized when the collision time Tc equals the idle time T1 within the measurement period.